Geochemical prospecting for subterranean accumulations of petroleum or natural gas based on analysis of collected soil gas samples is well known. Results of soil gas analysis provide information useful for evaluating the potential that a subterranean geologic structure may contain an accumulation of petroleum, natural gas or another targeted material. This information is helpful in selecting specific locations for further exploration evaluation, such as the drilling of exploration wells.
Various methods have been described for collecting and analyzing soil gas samples. One method for collecting and analyzing soil gas samples is described in U.S. Pat. No. 4,573,354. According to that method, soil gas samples are collected by adsorption of volatile soil gas compounds onto a carbon-coated ferromagnetic wire placed in holes in the ground. The adsorbed volatile compounds are then desorbed from each wire by heating the wire, using the Curie-point phenomenon. The desorbed compounds are swept into a mass spectrometer and a mass spectrum (referred to as a fingerprint) of the soil gas sample is obtained. A typical mass spectrum fingerprint of a soil gas sample collected above a petroleum reservoir is shown in FIG. 1.
The fingerprint provides some information concerning components of the soil gas sample. For example, mass peaks at m/z of 43, 57, 71, 85, and 99 have been shown to be associated with aliphatic hydrocarbons, while peaks at m/z of 91 and 105 have been shown to be associated with alkyl aromatic hydrocarbons. Also, a reference fingerprint obtained from an area over a known petroleum or gas reservoir may be compared with fingerprints of soil gas samples obtained from other geologically similar areas to identify exploration locations with a high potential for the presence of petroleum or natural gas. A preferred method of comparison according to the method of U.S. Pat. No. 4,573,354 is a multivariate statistical analysis, although other comparison techniques may also be used.
One problem that has been discovered with the method disclosed in U.S. Pat. No. 4,573,354 is that there is often one or more contaminant component(s) in the soil gas samples that may distort the mass spectrum, thereby impairing the accuracy with which the mass spectrum can be analyzed and, accordingly, the reliability of the analytical results as an indicator for the presence of the targeted material. In this regard, a contaminant component may be a naturally-occurring or human-introduced compound that is not associated with or indicative of underlying geologic features. For example, terpenes are naturally-occurring compounds that come from vegetation. The presence of terpenes in a soil gas sample can significantly interfere with accurate analysis of the soil gas mass spectrum. An example of a human-introduced contaminant is trichloroethane, which is widely found in aquifers due to its extensive use as a cleaning solvent. The presence of trichloroethane in a soil gas sample can significantly impair the accuracy of analysis. There is a significant need for a method to reduce the detrimental interference that may be caused by the presence of these and other contaminants.